1. Field of the Invention
The present invention relates to a switching power supply which outputs a direct current (DC) voltage.
2. Description of the Related Art
With a conventional flyback-type switching power supply, the switching operation of a switching element such as a field effect transistor (FET) for switching the primary side voltage of a transformer is controlled as follows. For example, the ON/OFF operation of the FET is controlled based on the secondary side voltage of the transformer, the voltage corresponding to the current flowing through the FET, and the voltage between the drain and source of the FET. To control this operation, an integrated circuit (IC) dedicated for switching operation control is used. Specifically, the FET is turned ON when the secondary side voltage exceeds a first reference voltage and the voltage between the drain and source of the FET is a second reference voltage or below. The FET is turned OFF when the secondary side voltage is lower than the voltage corresponding to the current flowing through the FET. Such a switching power supply uses a technique for reducing the power consumption at the time of light loading by shortening a period during which the FET is turned ON in the light load state (Japanese Patent Application Laid-Open No. 2000-148265).
In recent years, reducing the power consumption in the light load state has become one of problems for equipment mounted with a switching power supply. For example, by further reducing power consumption of the equipment mounted with the switching power supply in the light load state (also referred to as operation standby state), equipment having lower power consumption than conventional equipment can be provided. Particularly in the operation standby or non-operating state, in many cases, a power supply source such as a switching power supply consumes the most amount of power in an apparatus. Therefore, the necessity of further reducing power consumption of the switching power supply in the light load state is increasing.
One of factors which prevents the reduction in power consumption of the switching power supply in the light load state is switching loss of the FET (switching element). A possible method for reducing switching loss of the FET is to increase the amount of current to be sent to the FET while it is ON to reduce the number of switching operations per unit time.
However, increasing the amount of current to be sent to the switching element while it is ON to reduce the number of switching operations causes a problem that the transformer generates a beat sound. When the number of switching operations is reduced to increase the amount of current to be sent to the switching element while it is ON, a beat sound of the transformer increases. Specifically, when attempting to reduce switching loss in the light load state, a beat sound of the transformer is generated, which is harsh sound for a user. In the light load state in many cases, since the equipment is not operating, a beat sound is easily heard by the user.